iOS is Apple's OS for iTouch, iPhone and iPad devices.
You can develop programs on the iPhone simulator free of charge.
To be able to download iOS programs to actual iOS devices, you will need to join Apple's developer program which currently costs about $100USD per year.
To actually run a Monkey app on a real iDevice, you will need to manually open the output xcode project (.buid/ios/MonkeyGame.xcodeproj) in xcode, select 'device' as the target and build and run.
In-app purchases can be added to your app using the MonkeyStore class.
Testing IAP is a bit tricky. The general idea on both Android/iOS is that you create 'test' accounts to test with, and add these test accounts to GooglePlay Dev Console and/or iTunes Connect. Test are accounts are generally just email address/password combos, although on Android you'll have to provide more details the first time you use the test account.